Every bridge and associated port is in one of the following spanning tree states:
■ Disabled—The port is not participating in spanning tree and is not active.
■ Listening—The port has received data from the interface and will listen for frames. The bridge only receives data; it does not forward any frames to the interface or to other ports.
■ Learning—The bridge still discards incoming frames. The source address associated with the port is added to the CAM table. BPDUs are sent and received.
■ Forwarding—The port is fully operational; frames are sent and received.
■ Blocking—The port has been through the learning and listening states, and because this particular port is a dual path to the root bridge, the port is blocked to maintain a loop-free topology.
In some situations, you do not want spanning tree to go through the preceding steps (listening, learning, and forward/blocking, which can take up to 45 seconds) but rather to immediately enter a forwarding state. For example, a PC with a fast processor connected to a switch does not need to test for any BPDUs (PCs do not run spanning tree), and the port on the Ethernet switch should enter a forwarding state to allow the PC immediate connectivity. This feature is known as portfast on Cisco switches. To enable portfast, use the Catalyst command set spantree spantree-number portfast interface enable.
NOTE Concurrent routing and bridging/integrated routing and bridging, routing information fields, source-route bridging, and source-route translational bridging are not covered in the CCIE Security written exam, and they are not part of the blueprint.
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